Machines and molds for producing injection molded plastic articles of two or more layers of one or more different resins are well known. Such composite articles can be used in a wide variety of applications. Multi-materials can include materials with different molding properties, such as PET and PEN, the same material with various additives such as dyes, or combinations of these. For example, multi-layer multi-material articles which are injection molded include the keys used in personal computer keyboards, wherein the indicia of the function assigned to the key is formed from a different colored material than the remainder of the key and components such as multi-colored lenses used for the stop and turn signal indicator lights in automobiles.
Another common use is in the manufacture of multi-layer, multi-material articles for the packaging of food wherein, for example, U.S. FDA regulations require that only virgin plastic materials be employed in locations which contact the food. Generally, it is desired to reduce the amount of virgin material which is employed in such packages for both environmental reasons (wherein it is preferred to use recycled materials) and for cost reasons (as virgin material is more expensive than recycled plastic materials). Accordingly, multi-layer multi-material packages have been produced which include a first, thin, layer of virgin plastic material which contacts the food and a second, thicker, layer of recycled or otherwise less expensive material which is laminated to the first layer during injection molding to provide strength to the package.
One area where such multi-layer packaging is employed is in bottles and other vessels manufactured from PET or other materials. PET bottles and vessels are commonly blow molded from "preforms" in a well known manner, the preforms having been manufactured by injection molding to form a thread on the neck portion of the bottle to receive a bottle closure. It is known to form a multi-layer preform of PET, the inner most layer of which is virgin plastic and at least some portion of the remainder of the preform being recycled plastic material. In such cases, when the bottle or vessel is blow molded from the preform, the virgin material forms a continuous inner layer within the vessel and the recycled or other material surrounds the outside of the inner layer to increase the overall strength of the vessel to an acceptable level.
In other circumstances, the layers employed in multi-layer articles can have properties other than, or in addition to, being different colors and virgin and recycled materials, for example layers can have different chemical properties, etc. Also, more than two layers can be employed, if desired. It is known, for example, to produce a multi-layer preform for blow molding PET bottles and vessels wherein a layer of barrier material is located between the inner layer of virgin material and the recycled material, the barrier layer inhibiting take-up of CO.sub.2 gas from carbonated beverages stored in the blown bottle by the PET materials behind the barrier.
Various systems and techniques for molding multi-layer, multi-material plastic articles are known. Generally, such systems are based on either co-injection, over-molding and/or insert-molding systems. In all co-injection methods, the mold remains closed until the cavity is filled by the injection of two or more plastic materials into the cavity, either simultaneously or sequentially.
In sequential co-injection, a measured amount of a first material is injected into the cavity and an amount of a second material is then injected into the first material within the cavity. Due to a "skin" effect, the first material maintains its contact with the cavity walls and the second material pushes the first through the cavity, such that the materials fill the cavity with the second material sandwiched between inner and outer layers of the first.
In simultaneous co-injection, both materials are injected into the cavity at the same time, for at least part of the injection operation, and the differing viscosity, skin effects and other characteristics of the materials and the injection process result in the desired formation of layers of the materials within the cavity.
In the majority of co-injection methods, the article is made of maximum three different materials displaying different characteristics or/and functions. For example, one material can be a virgin resin, the second one can be a recycled version of same or different resin and the third can be a chemical barrier layer (such as EVOH, Nylon, MXD6) formed between them, or as a first layer. In common applications using two materials, an article can be formed having three or five layers (2M3L or 2M5L). If three materials are used, the article can have either three (3M3L) or five layers (3M5L).
Sequential co-injection systems for preforms are discussed in U.S. Pat. No. 4,781,954 to Krishnakumar et al. and U.S. Pat. No. 4,717,234 to Schad et al., the contents of each of which are incorporated herein by reference. A more recent co-injection system, shown in U.S. Pat. No. 5,582,788 to Collette et al., shows the use of a turret injection molding machine for co-injection which allows for improved cooling of molded articles.
Simultaneous co-injection systems for preforms are discussed in several U.S. Patents, such as those assigned to American National Can. Of interest in this regard in U.S. Pat. No. 5,523,045 to Kudert et al. which shows a multi-material co-injection nozzle design suitable for multi-layer preforms.
An innovative mold design capable of performing either simultaneous or sequential molding is described in U.S. Pat. No. 5,651,998 to Bertschi et al. and assigned to the assignee of the present invention. This application shows the first mold design wherein hot runner injection nozzles are located on the opposite sides of a cavity to inject two or more different resins. This approach simplifies the mold and allows for injecting into cavities which are arranged in a more compact, denser manner, as the nozzles for a single cavity are not on the same side of the mold.
While conventional co-injection methods offer some advantages as they use a single cavity and all the injection units are on one side of the injection molding machine, they also have several significant drawbacks. One of them is that it is difficult to obtain continuous and uniform layers of the different materials as they interact in a complete molten state and proper metering of the materials is often difficult. This is especially true when three materials are to injected. Further, the mold design and the hot runner design become very complicated as a single manifold or a single nozzle must be able to work with different materials having different processing parameters. These problems are further exacerbated for high cavitation molds, such as 48 or 96 cavity molds. Another difficulty is cooling, wherein thick articles require longer residence time in the mold close position, which affects the cycle time.
Some of the disadvantages of the co-injection systems are overcome by over-molding systems, where each injection operation is performed in a different mold cavity. Generally, the first injection operation is performed in a mold cavity to create the first layer of an article and the cavity is then changed to increase the volume and, commonly, to alter the geometry of the cavity space. Usually this is accomplished by changing the cavity and using the same core that holds the molded article. A second molding operation is then performed with the first layer of the article, which is retained by the core, being placed in the changed cavity. During the second injection the new molded material bonds to the previously molded layer in the mold to form the multi-layer article. As will be apparent, while the second cavity has a larger volume than the first, it will be understood by those of skill in the art that the actual cavity volume which must be filled in the second injection operation can be less than the volume filled in the first injection operation, with the balance of the volume being occupied by the first layer. As will also be apparent, over-molding can include more than one over molding operation to form articles using more than two resins and/or with more than two layers, if desired.
While good results can be obtained by over-molding, the necessity to open the mold to move a previously molded layer of an article to a second mold cavity for molding of the next layer has been difficult to achieve in a cost effective and reliable manner, especially if there are geometrical profile differences between the over-molded layers. U.S. Pat. No. 3,914,081 to Aoki shows an early attempt to perform over-molding employing a rotary stripper plate which is used to extract, hold and transfer a molded first layer of an article to a second mold cavity, wherein a second layer of resin of a different color is injected. U.S. Pat. No. 3,947,176 to Rainville shows a split mold design that allows ejection of the article after the molding of a threaded neck portion of the article by splitting the mold laterally. Rainville-type molds have proven to be difficult to manufacture, need more "real estate" to allow opening of the mold walls, present sealing problems over a greater area and tend to leave injection marks on the molded article.
Attempts to produce a more suitable over-molding system include U.S. Pat. No. 4,744,742 and U.S. Pat. No. 4,830,811 to Aoki which shows a two cavity mold design for preforms which is used with a rotary injection blow-molding machine. In these systems, the core enters a first cavity in which the first, inner, layer of a preform to molded. The core is then removed from the first cavity with the molded layer still in place and is inserted into a two portion second cavity, the lower portion of which is a single piece cavity of a larger diameter than the first and the upper portion of which is a two-part, split, cavity which defines threads for the neck portion of the preform. The second layer is then injected into the two portion cavity and the core is removed from within the cavity. The upper, threaded, portion of the cavity extracts the molded preform from the lower portion of the cavity and moves it to a blow molding station. After blow molding, the upper portion of the cavity is split to allow removal of the finished bottle.
The system taught by Aoki suffers from a number of disadvantages. First, the design is not readily applicable to forming more than two preforms per cycle, due to the complexity of the transfer platen used to move articles and the upper portions of the molds. Also, after the second injection operation is performed, the core is removed from the molded article prior to its transfer to the blow-molding station, preventing cooling of the interior of the preform by the core during the transfer. Thus, the bulk of the cooling must be performed before removal of the core, resulting in a relatively long cycle time.
A more recent attempt to produce over-molded preforms having a thread on the neck portion is shown in published European Patent Application 715,937 A1 to Massano. This reference teaches an injection mold to perform two-layer over-molding of a two-material PET preform wherein the mold comprises a stationary cavity plate, a moveable stripper/cavity and core plates. The cavity plate comprises adjacent pairs of single piece cavities of two different diameters and the stripper/cavity plate includes adjacent pairs of two-part cavity portion elements which can be split laterally. One cavity portion of each pair, which is aligned with the smaller diameter cavity in the cavity plate, has a smooth bore of the same diameter as the smaller diameter cavity and the other cavity portion of each pair, which is aligned with the larger diameter cavity, includes a thread to define the threaded neck portion of the preform.
The core plate has rotatable pairs of adjacent cores and molding is performed by inserting the pairs of cores into the pairs of cavities with the stripper/cavity plate contacting the cavity plate so that the cavity portions on the stripper/cavity plate form part of the cavity for the injection operation.
A complete injection operation is performed by injecting a first layer of material into the smaller diameter cavity and cavity portion, then the core and cavity/stripper plates are each moved away from the cavity plate until the end of the molded preform has been completely removed from the cavity, after which the core plate continues to move away from the cavity plate while the stripper/cavity plate remains in place. The core plate moves away from the now stationary stripper/cavity plate to remove the molded first layer, which remains on the core, from the smooth-bored cavity portion on the stripper/cavity plate. Just prior to the core being completely removed from the cavity portion, the two parts defining the pair of cavity portions are separated to allow the completed preform (commenced in the previous injection cycle) to fall from the threaded cavity portion, having been removed from the core by the engagement of the molded threads with the threaded cavity portion.
The molded first layer remains on the other core, being pulled through the smooth-bored cavity portion. Once the core and the molded first is completely removed from the smooth-bored cavity portion, the pair of cores are rotated one hundred and eighty degrees on the cavity plate so that the core with the molded first layer can now be inserted into the larger diameter cavity, through the threaded cavity portion on the stripper/cavity plate, and the now-empty other core can be inserted into the smaller diameter cavity through the smooth-bored cavity portion to commence another injection molding cycle.
The core plate and the stripper/cavity plate are closed to the cavity plate and the second layer is injected into the larger diameter cavity and the threaded cavity portion to complete the molding of the preform on this core (a first layer is injected into the other cavity with the smooth-bored cavity portion to commence the molding of the perform on that core). The cavity plate and stripper/cavity plate are then moved away from the cavity plate, as described above, to eject the completed preform and to rotate the cores for the next portion of the cycle.
The Massano system described above suffers from several disadvantages. In particular, the core plate must be moved away from the cavity plate for a distance exceeding at least twice the length of the molded articles while the stripper/cavity plate must be moved away from the cavity plate for a distance exceeding the length of the molded articles to allow ejection of the molded articles. These opening requirements result in a slower cycle time, while the plates move the required distances, and in a machine which requires a relatively large amount of floor space in which to operate. Also, the molded first layer is pulled through the smooth-bored cavity portion at the end of the first injection operation and this can result in damage to the molded first layer. Further, the requirement to rotate each pair of cores increases the expense of manufacturing the machine and can lead to leaking of cooling fluid from the cores, etc.
Published PCT patent application WO 97/02939 to Collette et al. shows two other injection molding machines for over-molding. The first machine shown is a turret machine with a number of cores mounted on each of a pair of opposed sides of the turret and a pair of cavity plates, each with a set of a corresponding number of cavities, facing each turret face. The first set of cavities is used to form the first layer of the molded article and the second set of cavities each including cavity extension portions to define the threads of a preform neck. One cavity plate and the turret move relative to the other cavity plate, and the turret rotates to move cores with a first molded layer from the first set of cavities to the second set of cavities where the second layer is molded with the cavity extension portions closed. The turret mold shown in Colette is used in conjunction with a conventional three platen injection molding machine. As shown in FIG. 2a of Colette, the second injection station unit (more exactly the second cavity plate) is located opposite the first one and in front of the clamping unit (not shown). The clamping unit thus prevents the injection unit from being located perpendicular to the mold plate, and instead it must be located at 90.degree. to the stroke of the clamping unit. This results in Collette's machine having a large total foot-print. Further, Collette system requires an additional ejection system on the core plate to eject the molded articles from the cores which have been retracted from the second set of cavities. Such ejections systems are expensive and/or difficult to provide and can introduce other problems in the molding operation, such as core shift.
The second machine taught in Collette is a shuttle-type system wherein the cavity plate has two sets of first cavities surrounding a set of second cavities and two sets of cores are mounted to a core plate which shuttles the cores between a first position, wherein the first set of cores is aligned with one set of first cavities and the second set of cores is aligned with the second set of cavities, and a second position, wherein the first set of cores is aligned with the second set of cavities and the second set of cores is aligned with the other set of first cavities. The core plate is laterally "shuttled" between the first and second positions each time the mold is opened to sequentially insert a core in one of the first sets of cavities, where a first layer is molded, and then in one of the second set of cavities where the second layer is molded. This machine suffers from disadvantages in that it requires an extra set of cavities, i.e. --three sets of cavities produce two sets of articles, which increases the expense of the mold.
Multi-layer articles can also be formed by insert-molding wherein an insert, formed by extrusion, injection molding, thermoforming, etc., is placed into a mold cavity and a layer of another material is then injected to fill the cavity. In fact, insert-molding can be combined with over-molding or co-injection to encase the insert between multiple layers of different materials, if desired.
It is desired to have an efficient, reliable and cost-effective injection molding machine and mold therefore to form multi-layer molded articles.